Drainage pattern and regional morphostructure at Melka Kunture (Upper Awash, Ethiopia)

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Drainage pattern and regional morphostructure at Melka Kunture (Upper Awash, Ethiopia)

Guillaume Bardin, Jean-Paul Raynal, Guy Kieffer

To cite this version:

Guillaume Bardin, Jean-Paul Raynal, Guy Kieffer. Drainage pattern and regional morphostructure

at Melka Kunture (Upper Awash, Ethiopia). Studies on the Early Paleolithic site of Melka Kunture,

Ethiopia - 2004. Edited by Jean Chavaillon and Marcello Piperno, Istituto Italiano di Preistoria e

Protostoria, pp.83-92, 2004. �halshs-00003987�

Geology, volcanology and geochemistry

Drainage pattern and regional morphostructure at Melka Kunture (Upper Awash, Ethiopia) ...83 Guillaume Bardin, Jean-Paul Raynal, Guy Kieffer

Volcanic markers in coarse alluvium at Melka Kunture (Upper Awash, Ethiopia) ...93 Guy Kieffer, Jean-Paul Raynal, Guillaume Bardin

Trace element geochemistry in Balchit obsidian (Upper Awash, Ethiopia) ...103 Gérard Poupeau, Guy Kieffer, Jean-Paul Raynal, Andy Milton, Sarah Delerue

Lithology, dynamism and volcanic successions at Melka Kunture (Upper Awash, Ethiopia) ...111 Jean-Paul Raynal, Guy Kieffer

Garba IV and the Melka Kunture Formation. A preliminary lithostratigraphic approach ...137

Jean-Paul Raynal, Guy Kieffer, Guillaume Bardin (with the collaboration of Geneviève Papy)

1. Résidence Parc des quatre seigneurs, 111 rue Francis Lopez, 34000 Montpellier, France. [email protected].

2. Université de Bordeaux 1, Institut de Préhistoire et de Géologie du Quaternaire, UMR 5199 CNRS, Avenue des Facultés, F- 33405 Talence et GDR 1122 CNRS, France. [email protected]. 3. UMR 6042 CNRS,

Université Blaise Pascal, Maison de la Recherche, 4 rue Ledru, 63057 Clermont-Ferrand Cedex 1, Centre de Recherches Volcanologiques et GDR 1122 CNRS.

Geology, volcanology and geochemistry

Drainage pattern and regional morphostructure at Melka Kunture (Upper Awash, Ethiopia)

Guillaume Bardin

, Jean-Paul Raynal

, Guy Kieffer

Melka Kunture depression belongs to the upper Awash basin. A village of the same name stands on the border of the Ethiopian Plateau, some fifty kilometres south of Addis Ababa (Fig. 1).

The basin surface is around 3000 km

and its altitude varies from 2000 to 2050 m. It is delimited by pliocene volcanoes the main ones being Wachacha and Furi in the north, Boti and Agoiabi in the south.

Upstream, the basin is a collapsed plain and downstream a less collapsed zone of enclosed gorges (Taieb 1974). Its eastern limit is marked by the main graben of the Ethiopian Rift belonging to the large East- African rift system (Mohr 1999). The Melka Kunture area is made up of valleys with inner terraces of which have resisted erosion. The visible thickness of those deposits is around 30 m, but the total thickness of the various levels is about 100 m.

The Melka Kunture area shows dissimilar morphology between the right and left banks of the Awash River. On the left bank, the plain goes up to the Wachacha slopes as rather regular glacis (Fig. 2), while the right bank shows a series of steps linked to a compartmentation formed by a Northeast-Southwest fault system. This segmentation affects the series of less collapsed compartments separating the Melka Kunture region from the rift graben.

Tectonic evolution of the Melka Kunture area determined the sedimentation and the morphological types. This evolution still develops today. The Holocene topography, regularised in wide glacis, is in com- plete imbalance with flows responsible for vigorous incisions and fast destruction of the pedological and vertisolic cover.

A morphostructural analysis (Bardin 2000) based on topographic maps was used (Howard 1967;

Prud’homme 1972; Griboulard and Prud’homme 1987; Collina-Girard 1989; Collina-Girard and Griboulard 1990). This technique considers base maps as useful objects that can reveal quite subtle mor- phology. Its aim is to extract the best possible information contained in cartographic documents, which must be the basis of any geographical, geological or geomorphological study.

Studies on the Early Paleolithic site of Melka Kunture, Ethiopia - 2004: 83-92.

G. Bardin, J.-P. Raynal, G. Kieffer 84

Fig. 1. Location map (after Woldegabriel et al. 1990).

Drainage pattern and regional morphostructure The regional drainage pattern

The regional drainage pattern is characterised by its heterogeneity but several different zones can be distinguished (Fig. 3). We use the classification of Howard (1967).

Zone 1 is characterised by a physiographical pattern of radial type. This configuration is linked to the presence in the zone of quaternary volcanoes such as Wachacha, Furi, and Wato Dalecha Mount. The drainage density is high.

In zone 2, the drainage density is low and we notice flow convergence zones as for example the Dilu or Haro-Dila plains, characterised by a centripetal pattern. In these intensely cultivated plains, many irrigation ditches have been built. Mount Debel is revealed by a regional drainage pattern of radial type. In this zone, the main river, the Awash, expands as it emerges from the Dilu plain and flows towards the south- east.

Zone 3 presents a high drainage density, with a dendritic pattern, one part of which converges towards the Aba Samuel Lake, another towards one of the Awash River’s large tributaries, the Atebella River, and a third towards the Awash River itself. The Awash River in the south and Wachacha, Debel and Furi volca- noes in the north mark its limits. The Awash River delimits zone 3 and 5. It still is the main drainage and flows east and southeast.

In zone 4, the pattern is organised in parallel sets flowing towards the north-west. This time, the Awash River is not the main drain, but the Watira River, which flows towards the north-east.

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Fig. 2. The Wachacha and its southwestern slopes. Cliché J.-P. Raynal

In zone 5, fluviatile networks are of flat lattice or directional lattice type with a main orientation towards the north-east, forking towards the southeast, and lying on the right side of a line passing through Bedesa and the exit of the Awash gorges.

In conclusion, the regional drainage pattern of the studied zone is defined by the large variety of its graphic characters. These take into account the topographic and geological structures and bring to the fore a dissimilarity between the morphology of the right and left banks of the Awash River. This is evidenced by a pattern of main drainage preferentially directed towards the northeast-southwest on the right bank and by a pattern more dependant on the topography (flowing on the slopes of quaternary volcanoes and convergence towards the plains) on the left bank. The Awash River is the main drainage feature. It flows either from west to east or northwest to southeast. Most of its tributaries flow in directions almost orthog- onal to its route. Dilu, Geber and Haro-Dila plains are grouped on an east-west line. They also function as reservoirs (convergence zones of the flows) and as a source of river supplies. The Awash River increases its strength as it flows through these plains.

Fig. 3. Subdivisons of the regional drainage pattern.

G. Bardin, J.-P. Raynal, G. Kieffer

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Drainage pattern and regional morphostructure Repartition and orientation of preferential directions, of obstacles to drainage

and of straight sections

This method postulates that surface drainage patterns are strictly dependant on the substratum anisotropy (faults, synclines, deep anticlyne). It highlights major directions of drainage, straight parts or abrupt changes of direction directly caused by obstacles in the drainage lines. The distribution and orien- tation are studied and help to characterize major structural directions which can be linked with the histo- ry of faulting.

Several preferential structural directions emerge (Fig. 4):

- An important W to E direction (N 90°) - A NE-SW direction (N 45°)

- A NS direction (N 180°)

- A NW-SE direction (N 130°-140°) - A SSW-NNE direction (N 22°)

Among these, we find again the two directions, which seem to guide the Awash River: N 90° and N 130°-140° and the N 45° direction. These directions are probably linked to a structural control of surface flows. They follow fault lines and fractures corresponding to different evolutionary phases of the Ethiopian Rift. We can link the N 90° direction to the W-E tectonic line crossing the Ethiopian Plateau close to N 9°

latitude (Ambo lineament). This structural direction coincides with the alignment of the Wato-Dalecha and Wachacha volcanoes. It is likely that the N 130°-140° direction influenced the cutting of the gorge of the Awash River. The N 45° direction is probably linked to a period of more recent structural alteration within the Ethiopian Rift. This period of tectonic activity affected the right bank of the Awash River, giv- ing a structure with the appearance of toppled blocks. The formation of fault steps on the SE border of the 87

Fig. 4. Orientation of preferential directions, of obstacles to drainage and of straight sections.

G. Bardin, J.-P. Raynal, G. Kieffer 88

Melka Kunture basin builds the latter into a half-graben. We also observe fault steps at the exit of the Awash gorges. These steps announce the arrival in the main graben of the Ethiopian Rift.

Study of the envelope surfaces

The method consists in retracing the contour lines without accounting for thalwegs or valley line floors.

Thus, we obtain a theoretical surface that corresponds to the topographic image before the development of the incision processes. Using a 1/50000 map, we do not include thalwegs under 3 (Collina-Girard 1989).

This representation brings to the fore the sub-rectilinear and oriented nick-points, indicating structur- al directions. Transferring these characteristic lines to tracing paper, we obtain a basic structure of the anisotropies of the sedimentary cover. In practice, this technique encounters the constraints of carto- graphic documents namely scales and the of contour line interval that affects the plotting precision. In the present case, the contour line interval on the 1/50000 map of Melka Kunture is 20 metres. For some areas with low differential relief, an interval of ten metres would allow a more refined plot.

We have limited this study to the Melka Kunture map alone, which seemed the most representative.

We can observe simultaneously the stepped border leading to the rift graben and the gorges upstream of the Melka Kunture basin (Fig. 5). Oriented nick points (little visible directly on the 1/50000 map) cer- tainly betray geological anisotropies of tectonic origin. Actually, these nick-points are oriented and organ- ised following preferential directions.

We can see a network of structural directions converging towards the NE. This network has segment- ed the Awash gorges. A structural line passes where the Awash enters into the gorge rather than at the alignment of the Melka Kunture fault, as we observed on the field. Several structural directions cross the Awash gorges creating nick points or changes of direction of its course. The exit of the gorges also corre- sponds to several structural directions. They produce the series of successive steps at the SE border of the raised tectonic block separating the Melka Kunture basin from the main graben of the Ethiopian Rift.

Longitudinal profile of the Awash River

Producing a longitudinal profile aims to bring to the fore the equilibrium or desequilibrious state of a river. It allows one to follow the river’s evolution in accordance with its altitude and to gather limits of ele- vations that correspond with tectonic anisotropies or with size limits for various blocks. According to the variations in depth of the river in the bedrock, one can deduce the nature of the rock, different sediment types, differential erosion or calcareous marls etc.

From upstream to downstream, we can divide the profile into four parts, of which only the fourth has

a profile close to equilibrium (Fig. 6). We notice important nick-points at kilometres 56 and 70. In the

first part of the profile, the Awash flows on a hard surface, probably eruptive rocks. Then, we observe an

abrupt threshold, as if the river reached a more collapsed tectonic block constituted of the same rocks. At

the commencement of the gorges, the Awash is suddenly enclosed in the substratum, firstly coming up

against hard volcanic roots (small volcanic blocks of Muti and Tinishu Muti and a lava lake) and against

the front of the quaternary lava flows (Fig. 7). This explains the rounded and irregular aspect of the pro-

file. From kilometre 75, the profile regularises.

Drainage pattern and regional morphostructure 89

Fig. 5. Study of the envelope surfaces.

Fig. 6. Longitudinal profile of the Awash River.

G. Bardin, J.-P. Raynal, G. Kieffer 90

Structural sketch of the Melka Kunture region

The results of the morphostructural analysis and photo-interpretation allow us to build a structural sketch of the Melka Kunture region (Fig. 8). From the characteristics of the drainage pattern, it has been pos- sible to determine structural directions coinciding with those obtained with the study of envelope surfaces.

We find again the tectonic lines of W-E direction on which the Wato-Dalecha, Wachacha, Furi and Debel volcanoes are aligned (Ambo lineament). These volcanoes perhaps appeared because of such faults during an ancient tectonic phase, corresponding with the commencement of the Ethiopian Rift event. This direction is still seismicaly active (Gouin 1979). As well, flow convergence zones follow those structural directions.

The NW-SE directions, which characterizes transverse faults zones in the Main Ethiopian Rift, are posterior to the tectonic phase that gave rise to the W-E faults. These NW-SE directions determine the course of the Awash River, and notably its gorges.

The SW-NE directions correspond to one or more tectonic phases posterior to those responsible for W- E and NW-SE directions. These faults formed the SE border of the Melka Kunture basin as a half-graben (Figs. 8, 9). The Melka Kunture fault belongs to the northern part of the Guraghe major border faults stepped system in this western part of the Main Ethiopian Rift. Between the SE border of the basin and the toppled blocks forming the main graben of the rift, stands a raised block, which the Awash encloses.

This block is crossed by a network of structural lines converging towards the NE. One of those lines caus-

Fig. 7. Awash River gorges and Tinishu Muti volcano in the background. Cliché M. Hirbec-Raynal

Drainage pattern and regional morphostructure es the initial formation of the gorge and another finishes the incision. Actually, as we can see in the field, the gorges do not begin immediately at the Melka Kunture fault level, but further afield at the level of another structural line.

The Awash River, which is the main actor in the play of sedimentation in the Melka Kunture basin, gained most of its structural characteristics during the oldest tectonic phases (WE and NW-SE structural directions) different to those producing the structural directions that control the majority of surface flows (SW-NE direction). This can easily be seen on the right bank of the Awash River. At the time of its entrenchment, the Awash River certainly took advantage of an ancient NW-SE fault, which explains the general orientation of the gorges. At their commencement, the river came up against hard volcanic roots (namely a lake of aphyric lava) and against the front of quaternary lava flows. This explains the sudden nick-points visible on the longitudinal profile. At the contact with softer rocks, the profile regularises.

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Fig. 8. Structural sketch of the Melka Kunture region.

G. Bardin, J.-P. Raynal, G. Kieffer 92

Future research and new geological and geochronological information will bring more detail about the structural evolution of the Melka Kunture basin and its relationships with the Main Ethiopian Rift tectonic history.

Maps

- 1/250000: Akaki Beseka (Ethiopia) by Ethiopian Mapping Authority. EMA series, sheet NC 37-14, edition 2.

- 1/50000: Addis Abeba SW, Melka Kunture, Bantu and Teji, SMD 4 (DOS 450) series, sheets and editions: 0838 BI; 1 SMD/DOS 1973, 0838 B3; 1 SMD/DOS 1975, 0838 A4; 1 SMD/DOS 1975, 0838 A2; 1 SMD/DOS 1975. pub- lished by the Ethiopian Government’s Ministry of Land Reform and Administration (Survey and Mapping Department).

Acknowledgements

Financial support for field work and laboratory analyses was provided by CNRS (GDR Hommes et volcans avant l’histoire), Région Aquitaine and Région Auvergne for project Espaces volcaniques préhistoriques and Italian Archaeological Mission at Melka Kunture. We are grateful to Marianne Hirbec-Raynal for the English translation and to Peter Bindon for its revision We thank David Lefèvre for his support at University Paul Valéry Montpellier III and Jacques Collina-Girard for his unvalu- able contribution in data treatment.

Fig. 9. SW-NE border fault at Godeti. Cliché G. Kieffer